This invention relates to the measurement of a reference length of a running web and is more particularly concerned with the measurement of "board length."
In the manufacture of certain types of multi-layer printed documents, it is common practice to provide a row of punched holes along a longitudinal edge of a paper web (e.g., holes at half inch intervals) which are later used for collation of the sheets. During the punching operation, which may be performed concurrently with the printing operation, the paper is under tension. After the paper leaves the press, it relaxes somewhat, resulting in a small dimensional change in hole spacing and paper length. This change is dependent upon such factors as infeed tension in the press and the width, thickness, and modulus of elasticity of the paper.
Although the dimensional changes are small, for example. +0.03%, the error in alignment between holes in the various sheet levels as the sheets are collated accumulates and is proportional to the length of paper being collated. Collators can compensate for some of the differences in hole spacing, but when the variance becomes excessive, rolls of paper may be unusable.
In order to assure uniformity of hole spacing at the various sheet levels, it is common practice to measure a reference length, termed a "board length" or "throw length," and to determine the deviation of the measured length from a standard. Heretofore, the board length has been determined by the following procedure. First the press is started and brought up to full operating speed. Then it is stopped and a ten foot section of paper is removed. This test sample is placed on a ten foot ruler and the distance between 240 holes (approximately ten feet) is measured. Based on this measurement, the operator adjusts the press tension to correct any offsets from the standard ten feet. A complete run is then made. The hope is that at a constant speed the hole spacing will not vary. While this is often the case, changes in paper quality, press tension, and press speed, or interruption of the operation of the press may change the board length. When board length varies and remains uncompensated, large amounts of paper may have to be rejected. Thus, a suitable apparatus and method for measuring board length continuously, i.e., on-line, is highly desirable.